In photolithography, one step in the production of a microdevice, use is made of an exposure apparatus for projection exposure of an image of a pattern of a photomask or reticle on to a substrate for exposure (semiconductor wafer or glass plate coated with a photoresist). Recently, to deal with the increasing size of exposure areas accompanying the larger size of substrates etc., a stepping type exposure apparatus designed to divide the exposure areas of substrates into a plurality of unit areas (hereinafter sometimes called “shots” or “shot areas”) and successively projecting and exposing corresponding images of patterns on the shots has been developed.
In such an exposure apparatus, sometimes mismatch occurs at the stitched portions of the shots due to aberration of the lenses of the projection optical system, positioning error of the mask or substrate, etc., so exposure is performed while overlaying part of the image of the pattern for one shot and the part of the image of the pattern for another shot adjoining the same. At the overlaid parts of the patterns, the amount of exposure becomes larger than the portions other than the overlaid parts, so for example the pattern width (width of lines or spaces) at the overlaid parts of the patterns formed on the substrate becomes thinner or thicker in accordance with the characteristic of the photoresist.
Therefore, the distribution of the amount of exposure of the portions becoming the overlaid parts of the shots is set in a sloping manner so that it becomes smaller the more to the outside, the amount of exposure of the overlaid parts is made equal, by two exposures, to the amount of exposure of the portions other than the overlaid parts, and therefore a change in pattern width at the overlaid parts is prevented.
As the technique for realizing a sloping distribution of the amount of exposure at the overlaid parts of the shots, it is known to form a light attenuating part for limiting in a slope the amount of transmitted light at portions of the reticle itself corresponding to the overlaid parts. Formation of a light attenuating part at the reticle itself, however, increases the number of steps of production and cost of the reticle and raises the costs of manufacturing a microdevice etc.
Therefore, a technique of providing a density filter comprised of a glass plate formed with a light attenuating part similar to the above at a position substantially conjugate with the pattern formation surface of the reticle or a technique of providing a blind mechanism having a light blocking plate (blind) able to advance and retract with respect to the optical path at a position substantially conjugate with the pattern formation surface of the reticle and advancing or retracting the light blocking plate during exposure of the substrate so as to realize such a sloping distribution of the amount of exposure have been developed.
However, it is necessary to execute the exposure processing in a state with the substrate and reticle accurately matched in relative positions, so the reticle is aligned with the substrate via a predetermined reference by a rotational function and translational function provided by a stage holding the same. This alignment is sometimes performed for each exposure of the shots.
In a conventional exposure apparatus employing a density filter, however, while the posture of the density filter can be initially adjusted, it is fixed after this initial adjustment, so when the reticle is rotated or moved translationally for alignment, a mismatch occurs with the posture of the density filter, positional deviation occurs between the portions to be reduced in light at the pattern formation surface of the mask (portions corresponding to portions forming overlaid parts on the substrate) and the image of a light attenuating part of the density filter on the pattern formation surface of the reticle, the amounts of exposure of the overlaid parts of the shots and the portions other than the overlaid parts do not become uniform, and sometimes the widths of the pattern formed on the substrate do not become uniform.
Further, since conventionally the density filter is placed at a position substantially conjugate with the pattern formation surface of the reticle in the illumination optical system, the shape of the image of the light attenuating part of the density filter at the pattern formation surface of the reticle becomes warped etc. due to distortion or other aberration of the optical system including the lenses etc. interposed between the density filter and reticle, magnification error, etc., deviations in shape occur between the portions to be reduced in light at the pattern formation surface of the mask (portions corresponding to portions forming overlaid parts on the substrate) and the image of the light attenuating part of the density filter on the pattern formation surface of the reticle, the amounts of exposure of the overlaid parts of the shots and the portions other than the overlaid parts do not become uniform, and sometimes the widths of the pattern formed on the substrate do not become uniform.
Due to the above, there was the problem that the continuity or periodicity of the patterns formed on the substrate deteriorates and the characteristics or quality of the microdevices or photomasks produced are degraded.